The present invention relates to a method for controlling to keep off defects on magnetic disks, and more particularly to a record-arrangement control method for keeping off defects on magnetic disks which method is used to access a plurality of tracks with the aid of a channel program made up of a plurality of channel command words, and to write data in or read data out of the tracks at high speed in spite of defects on the tracks, thereby increasing the throughput of a data processor.
In a conventional magnetic disk apparatus, as described in, for example, a Japanese patent application Japanese Patent Unexamined Publication No. 49-52,612, the first area of each of tracks on a magnetic disk usually contains the positional information about defects on the track, and records are arranged on the track so that area of the track which contains a defect and has a predetermined length, is left unused, that is, the records are arranged so as to keep off the defect.
A disk access method has been known, in which one of a plurality of tracks are changed over to another track by a channel program made up of a plurality of channel command words to access the tracks efficiently. In this method, a dummy record corresponding to a changeover time (namely, a time necessary for changing a track over to another track) is provided on each track, and a track is changed over to another track at the dummy record. However, the dummy record contains no information, and hence is a useless area. Accordingly, it is desirable to make the length of the dummy record as short as possible. Thus, the length of the dummy record is made accurately correspond to the changeover time, that is, has no margin.
In a case where each of tracks belonging to a cylinder has no defect and all of the tracks have the same record arrangement, when the access to a record on one of the tracks has been completed, the next record on another track can be smoothly accessed by a track changeover operation, because the record on one track is spaced apart from the next record on another track by a distance corresponding to the changeover time in the circumferential direction of one track. However, in a case where a defect is present on a track, where a record on the defective track is separated into a plurality of parts of displaced so as to keep off the defect, and where the record on the defective track and the next record on a non-defective track (in which records are arranged at normal positions) are successively accessed by the track changeover operation, the distance between the record on the defective track and the next record on the non-defective track in the circumferential direction of the track may become shorter than the distance corresponding to the track changeover time. Accordingly, it may occur that the top of the next record on the non-defective track cannot be accessed immediately after the track changeover operation has been completed, but can be accessed after a magnetic disk having the non-defective track has made one revolution. In this case, there arises a problem that an access time is extended.